Abstract The first snake craniomandibular fossil specimen from the Campanian–Maastrichtian of South America is described herein. The specimen, from the Allen Formation in Rio Negro Province, consists of an incomplete anterior portion of a left dentary. The general morphology of the bone resembles that of the basal legged snake Najash Apesteguia and Zaher and the small madtsoiids from the Eocene–Miocene of Australia. The dentary was found associated with several isolated snake vertebrae, most of which are referable to madtsoiids, thus it probably belongs to one of the already known madtsoiid taxa.